CN1246509C - A method of laser crystallization - Google Patents

Abstract

The invention firstly forms at least one amorphous silicon island on a substrate, then carries out a first stage and a second stage of laser crystallization process in sequence, namely, firstly uses laser pulse with first energy density to irradiate the amorphous silicon island so as to crystallize the edge part of the amorphous silicon island into a polysilicon structure, and then uses laser pulse with second energy density to irradiate the amorphous silicon island so as to crystallize the central part of the amorphous silicon island into the polysilicon structure.

Description

A method of laser crystallization

technical field

The present invention provides a laser crystallization (laser crystallization, LC) method, especially a two-step laser crystallization method that can increase the process window (process window).

Background technique

In today's flat panel display products, liquid crystal display (LCD) is one of the most popular technologies, which are used in mobile phones, digital cameras, video cameras, notebook computers and even monitors that are common in daily life. Goods produced by technology. With the improvement of people's requirements for display visual experience and the continuous expansion of new technology application fields, flat-panel displays with higher image quality, high resolution, high brightness and low price will become the trend of future display technology development. Created the driving force behind the development of new display technology. The low temperature polysilicon thin film transistor (LTPSTFT) liquid crystal display (LCD) in the flat panel display has the characteristics of conforming to the trend of active drive (actively drive), and its technology is also a technology that can achieve the above-mentioned An important technological breakthrough for the target. In particular, it has the advantages of integrating metal oxide semiconductors and low-temperature polysilicon thin film transistors into the same process technology, so that the goal of system on panel (SOP) can be realized, so it has become an active goal of various manufacturers. object of research and development.

However, in the production process of low-temperature polysilicon thin film transistor liquid crystal display, since the heat resistance of the general glass substrate can only reach about 600°C, if the polysilicon film is directly fabricated at high temperature, the glass substrate will be distorted and deformed. Therefore, the traditional Polysilicon thin-film transistor liquid crystal displays often have to use expensive quartz as the substrate, so the scope of application can only be limited to small-sized liquid crystal panels. At present, another low-temperature polysilicon thin film production method using amorphous silicon thin film (a-Si thin film) recrystallization has emerged and has become the mainstream. Among them, excimer laser annealing (excimer laser annealing, ELA ) process has received special attention.

Please refer to FIG. 1 . FIG. 1 is a schematic diagram of a method for fabricating a polysilicon thin film by an excimer laser annealing process. As shown in Figure 1, at first on the glass substrate 10, deposit the amorphous silicon thin film 12 that thickness is about 500 angstroms (Å), then put the glass substrate 10 in a closed reaction chamber (not shown), to carry out excitation Excimer laser annealing process. Wherein, there are many kinds of methods for depositing the amorphous silicon thin film 12, such as low-pressure chemical vapor deposition (LPCVD), plasma-assisted chemical vapor deposition (PECVD) and sputtering (sputtering), and the excimer laser annealing During the process, the laser pulse 14 of the excimer laser can be irradiated from the transparent window (not shown) above the reaction chamber (not shown) to the amorphous silicon film 12 on the surface of the glass substrate 10, and the A method similar to scanning (scan) gradually sweeps all regions within the process range to rapidly heat the amorphous silicon film 12 within the process range to recrystallize it into a polysilicon thin film (polysilicon thinfilm, not shown).

Because in the process of excimer laser annealing, the amorphous silicon thin film will achieve rapid melting and recrystallization through the absorption of laser deep ultraviolet light, and the rapid absorption caused by this short-time pulse laser will only affect non-crystalline silicon. The surface of the crystalline silicon film is affected, so the glass substrate can be kept at a low temperature without being affected. Generally speaking, the commonly used types of excimer lasers include XeCl laser, ArF laser, KrF laser and XeF laser, etc. Different molecules will produce different wavelengths, and the output energy density will be based on different The thickness adjustment of the crystalline silicon film, taking an amorphous silicon film with a thickness of 500 angstroms as an example, the energy density output by the excimer laser is about 200 to 400 mJ/cm ² . After the excimer laser annealing process is completed, the rest of the subsequent liquid crystal display panel process can be further carried out, using the polysilicon film as the channel (source) or source/drain (source/drain) in the liquid crystal display, To form a driving circuit or a logic circuit in a liquid crystal display panel.

As mentioned above, since the quality of the amorphous silicon film 12 has a great influence on the characteristics of the polysilicon film formed subsequently, each parameter (parameter) in the deposition process of the amorphous silicon film needs to be strictly controlled in order to form a low polysilicon film. Amorphous silicon thin film with hydrogen content, high thickness uniformity and low surface roughness. In addition, in the process of the recrystallization of the amorphous silicon film 12 into a polysilicon film, many variables (variables) will have a direct impact on the grain size (grain size) and distribution (distribution) after the recrystallization is completed, and when the laser When inhomogeneity occurs during the crystallization process, various defects often occur.

Please refer to FIG. 2 , which is a schematic diagram of the energy density of the laser crystallization process in the prior art. As shown in Figure 2, when using known technology for laser crystallization process, the selected laser energy density E is between near-completely-melted energy density ( _ENCM ) and super lateral growth energy density (SLG energy density, E _SLG ). It can be clearly seen from Figure 2 that when the energy density of the laser is less than the energy density of nearly complete melting, since the energy density is not enough to supply the seed crystal (seed) to grow into a large grain (large grain), the formed grains are relatively small. Small; when the energy density is greater than the amorphous siliconization energy density (amorphousization energy density, E _α ), although the amorphous silicon film 12 can be completely melted, the crystallization method is achieved by quenching, so uniform nucleation is formed (homogeneous nucleation) phenomenon, also because of uniform nucleation, nucleation points will be generated everywhere, and the crystal grains cannot grow effectively, so the size of the formed grains will suddenly decrease, and even amorphous silicidation; and when the energy density When the energy density between super lateral growth and amorphous silicidation is between, although large grains can still be formed, small grains are also beginning to be produced, often resulting in a difference in grain size between different components (device to device) A situation where uniformity cannot be well controlled, resulting in electrical differences between different components.

The known methods for laser crystallization, however, have significant limitations. Please refer to FIG. 3A and FIG. 3B . FIG. 3A and FIG. 3B are schematic cross-sectional views of amorphous silicon islands 20A and 20B after laser crystallization in the prior art. Since usually in the actual manufacturing process, after forming the amorphous silicon layer 12, a photo-etching process (photo-etching-process, PEP, not shown) is also included to etch the amorphous silicon film 12 as shown in FIG. 3A or FIG. 3B shows amorphous silicon islands (amorphous silicon island) 20A, 20B. The amorphous silicon islands 20A, 20B may have different shapes according to the requirements of manufacturing process and design, and in FIG. 3A and FIG. The situation of the active area (active area) will be described.

As shown in FIG. 3A and FIG. 3B , due to the somewhat different heat dissipation directions, the thermal conduction rate (thermal conduction rate) of the edge portions 22A, 22B is greater than that of the central portion 24A, 24B, thereby forming a temperature gradient. Therefore, the amorphous silicon thin films of the edge portions 22A, 22B of the amorphous silicon islands 20A, 20B are first solidified (solidify) after reaching a nearly completely molten state, and then the residual (residual) in the amorphous silicon thin films of the edge portions 22A, 22B is formed. Amorphous silicon seeds (seeds, not shown) grow laterally toward the central portions 24A, 24B to become large grains 26A, 26B. But in any case, the speed of lateral growth has a certain limit, and usually only grows to 1-2 microns (μm). As shown in FIG. 3A , when the channel width of the device is small, the large grain 26A can grow to the center of the channel width, thereby improving the electrical properties of the device. Therefore, in the known method of first forming amorphous silicon islands and then laser crystallization, energy density higher than E _SLG is sometimes selected to increase the driving force of lateral growth. But as shown in FIG. 3B, when the channel width of the element is large, only the large crystal grain 26B grows in the edge part 22B, but grows into the phenomenon of the small crystal grain 28 in the central part 24B, and finally causes the element electrical property Degrade.

At the same time, the laser energy density range used in the known technology is too small, and when there is a slight error in the size of the laser energy density, it is easy to exceed the above laser energy density range. Even when variables such as the uniformity of the spatial distribution of laser energy, the degree of overlap of laser pulses (overlap), the temperature of the substrate during laser annealing, and the surrounding atmosphere (atmosphere) are not properly controlled, the It will relatively cause the laser energy density used to exceed the above laser energy density range.

Therefore, how can a new laser crystallization method be developed, which can not only promote the effective lateral growth of various parts of the amorphous silicon island to form uniform large grains, but also increase the process window of the laser crystallization process (process window) has become a very important topic.

Contents of the invention

The main purpose of the present invention is to provide a laser crystallization (laser crystallization, LC) process method, especially a two-stage laser crystallization method that can significantly increase the process window (process window).

In a preferred embodiment of the present invention, a substrate is provided first, and then at least one amorphous silicon island is formed on the substrate, and then the first-stage laser crystallization process is performed, and the amorphous silicon island is irradiated with a laser pulse having a first energy density. crystalline silicon island (amorphous silicon island, α-Si island), so that the edge portion of the amorphous silicon island is recrystallized into a lateral growth polysilicon structure (lateral growth polysilicon structure), followed by the second stage of laser crystallization process, using Laser pulses of a second energy density irradiate the amorphous silicon island to recrystallize a center portion of the amorphous silicon island into a polysilicon structure.

Because the laser crystallization method of the present invention utilizes a two-stage laser crystallization process (lasercrystallization, process, LC process), the edge portion of the amorphous silicon island is first recrystallized into large grains, and then the amorphous silicon island is recrystallized Small grains in the central portion are repaired to become large grains. Even when the amorphous silicon island is applied to a device with a large channel width, when the crystal grain grows laterally to the central part of the amorphous silicon island, there will be no growth failure due to the limit of the lateral growth speed. To the center of the line width, it is possible to fully avoid the electrical degradation of the element. At the same time, because only the central part of the amorphous silicon island needs to be considered during the laser crystallization process in the second stage, the process window of the laser crystallization method of the present invention can be significantly enlarged, so that there is no slight error in the size of the laser energy density , or other variables in the laser crystallization process are not properly controlled, it is easy to exceed the laser fluence range of the process. When the method of the invention is used in an actual production line, large-channel components with good electrical properties can be produced.

Description of drawings

FIG. 1 is a schematic diagram of a method for fabricating a polysilicon thin film by an excimer laser annealing process.

FIG. 2 is a schematic diagram of the energy density of the laser crystallization process in the prior art.

3A and 3B are schematic diagrams of cross-sectional results of laser crystallization of amorphous silicon islands in the prior art.

4 to 5 are schematic diagrams of the laser crystallization method in the present invention.

6A and 6B are schematic diagrams of cross-sectional results of amorphous silicon islands after laser crystallization in the method of the present invention.

FIG. 7 is a schematic diagram of the mobility of an N-type thin film transistor fabricated by the laser crystallization method of the present invention.

FIG. 8 is a schematic diagram of the initial voltage of an N-type thin film transistor fabricated by the laser crystallization method of the present invention.

Illustration of symbols

10 Glass substrate 12 Amorphous silicon film

14 laser pulses 20A, 20B amorphous silicon island

22A, 22B edge part 24A, 24B central part

26A, 26B large grain 28 small grain

100 insulating substrate 102 amorphous silicon island

103 laser pulses 104 edge parts

106 central part 108 large grains

111 small grain 112SLG grain

Detailed ways

Please refer to FIG. 4 to FIG. 6B . FIG. 4 to FIG. 5 are schematic diagrams of the laser crystallization method in the present invention, and FIG. 6A to FIG. 6B are schematic cross-sectional results of the laser crystallization of the amorphous silicon island 102 in the present invention. As shown in FIG. 4 , an insulating substrate 100 is provided first, and the insulating substrate 100 includes a glass substrate, a quartz substrate or a plastic substrate. Then an amorphous silicon thin film (amorphous silicon thin film, α-Si thin film, not shown) is formed on the insulating substrate 100, and then a photo-etching-process (PEP) is performed to form the amorphous silicon thin film (not shown). shown) are etched into amorphous silicon islands (amorphous island) 102 . The method for forming the amorphous silicon film (not shown) includes a low pressure chemical vapor deposition (LPCVD) process, a plasma assisted chemical vapor deposition (PECVD) process or a sputtering process. At the same time, the amorphous silicon island 102 may have different shapes according to the requirements of the manufacturing process and design, and in FIG. Situation to illustrate.

Then the insulating substrate 100 is put into a closed reaction chamber (not shown) to carry out excimer laser annealing process. During the excimer laser annealing process, the laser pulse 103 of the excimer laser can be irradiated from the transparent window (not shown) above the reaction chamber (not shown) to the amorphous silicon island 102 on the surface of the glass substrate 100, and According to a pre-set process range, all regions within the process range are scanned step by step in a scan-like manner, so as to rapidly heat the amorphous silicon island 102 within the process range. Meanwhile, the excimer laser includes XeCl laser, ArF laser, KrF laser or XeF laser.

The laser crystallization method of the present invention utilizes a two-step laser crystallization process to process the amorphous silicon island 102 . As shown in FIG. 5, when the method of the present invention carries out the laser crystallization process of the first stage (first step), the selected laser energy density _E1 is greater than (higher than) the super lateral growth energy density of the amorphous silicon island 102 ( super lateral growth energy density, E _SLG ), and in fact, it is preferable that the first energy density (first energy density) is greater than the amorphous siliconization energy density (amorphousization energy density, E _α ) of the amorphous silicon island 102 . As shown in FIG. 6A, since the heat conduction rate of the edge portion 104 of the amorphous silicon island 102 is greater than that of the central portion 106, when the first stage of the laser crystallization process is performed, a temperature gradient is generated. At this time, the amorphous silicon island 102 The edge portion (edge portion) 104 of the amorphous silicon island 102 in the edge portion 104 is first solidified (solidify) after reaching a completely melted (completely-melted) state, and then the amorphous silicon The central portion 106 of the island 102 grows laterally into at least one large grain 108 .

Since the lateral growth speed has a certain limit, the final size of the large crystal grains 108 is about 1-2 micrometers (μm). At the same time, the central portion 106 of the amorphous silicon island 102 is crystallized by quenching after reaching a completely molten state, because the central portion 106 of the amorphous silicon island 102 is everywhere due to homogeneous nucleation. There will be nucleation sites (nucleation sites), the grains cannot grow effectively, and the size of the formed grains will suddenly decrease, becoming many small grains111, and even amorphous silicidation, which cannot be effectively recrystallized (recrystallized) and maintained ( remain) in an amorphous silicon structure (not shown).

Then carry out the laser crystallization process of the second stage, when the method of the present invention carries out the laser crystallization process of the second stage (second step), the selected laser energy density E ₂ is not greater than (not greater than) the super side of the amorphous silicon island 102 In fact, the preferred second energy density is between (in between) the almost completely melted energy density (nearly-completely-melted energy density, E _NCM ) of the amorphous silicon island 102 and the super lateral growth energy density (super lateral growth energy density, E _SLG ).

As shown in FIG. 6B, at this time, the central portion 106 of the amorphous silicon island 102 has reached a nearly completely molten state, thereby making the central portion 106 of the amorphous silicon island 102 grow into a normal SLG grain 112 (about 0.2-0.5 μm crystal size) . The large grains that have been formed at the edge are hardly affected. Therefore, after the two-stage laser crystallization process, the microstructure of the amorphous silicon island 102 is composed of large grains 108 and normal SLG grains 112 , and its electrical properties have also been significantly improved. Please refer to FIG. 7 and FIG. 8. FIG. 7 is a schematic diagram of the mobility (mobility) of an N-type thin film transistor produced by the laser crystallization method of the present invention, and FIG. 8 is an N-type thin film produced by the laser crystallization method of the present invention. Schematic diagram of the threshold voltage of the transistor. It can be seen from Figures 7 to 8 that after the two-stage laser crystallization process, both the mobility of electrons in the channel and the initial voltage of the N-type thin film transistor have been improved to a considerable extent, especially when The improvement was more pronounced when the first laser crystallization process was implemented in the high energy density range.

The method of the laser crystallization process of the present invention utilizes a two-stage laser crystallization process to recrystallize the edge portion and the central portion of the amorphous silicon island into large grains respectively. In this way, the situation in the known technology that the grains derived from the amorphous silicon island cannot grow to the center of the line width due to the difference in heat conduction rate between the edge part and the central part and the limit of the lateral growth speed can be effectively utilized. avoid. And only need to consider the central part of the amorphous silicon island during the laser crystallization process in the second stage, the process window of the laser crystallization method of the present invention can be significantly enlarged, thereby improving the size of the laser energy density in the known technology When there is a slight error, or when other variables in the laser crystallization process are not properly controlled, the laser fluence range of the process can be easily exceeded. When the method of the invention is used in an actual production line, large-channel components with good electrical properties can be produced.

For the known laser crystallization method, the laser crystallization method of the present invention utilizes a two-stage laser crystallization process, first recrystallizes the edge portion of the amorphous silicon island into large grains, and then recrystallizes the central portion of the amorphous silicon island Small grains within the part are repaired to become normal grains. Therefore, even if the amorphous silicon island is applied to a device with a large channel width, when the crystal grains grow laterally to the central part of the amorphous silicon island, there will be no growth failure due to the limit of the lateral growth speed. To the center of the line width, thereby avoiding the electrical degradation of the component. At the same time, only the central part of the amorphous silicon island needs to be considered when performing the laser crystallization process in the third stage, and the process window of the laser crystallization method of the present invention can be significantly enlarged, so that there is no slight error in the size of the laser energy density , it easily exceeds the laser fluence range of the process. In addition, when other variables in the laser crystallization process are not properly controlled, it is relatively less likely to cause the laser fluence used to exceed the laser fluence range of the process.

The above descriptions are only preferred embodiments of the present invention, and all equivalent changes and modifications made according to the claims of the present invention shall fall within the scope of the patent of the present invention.

Claims (24)

1. A method for laser crystallization, the method may further comprise the steps: provide the basis; forming at least one amorphous silicon island over the substrate; performing a first-stage laser crystallization process, irradiating the amorphous silicon island with a laser pulse having a first energy density, so that the edge portion of the amorphous silicon island is recrystallized into a laterally grown polysilicon structure; and The second-stage laser crystallization process is performed, and the amorphous silicon island is irradiated with laser pulses having a second energy density, so that the central part of the amorphous silicon island is recrystallized into a polysilicon structure. 2. The method of claim 1, wherein the substrate comprises a glass substrate, a quartz substrate, or a plastic substrate. 3. The method of claim 1, wherein the method of forming the amorphous silicon island further comprises the following steps: forming an amorphous silicon film over the substrate; and A photoetching process is performed to etch the amorphous silicon film into the amorphous silicon island. 4. The method of claim 3, wherein the method of forming the amorphous silicon film comprises a low pressure chemical vapor deposition process, a plasma assisted chemical vapor deposition process, and a sputtering process. 5. The method of claim 1, wherein the laser is an excimer laser. 6. The method of claim 5, wherein the excimer laser comprises XeCl laser, ArF laser, KrF laser or XeF laser. 7. The method as claimed in claim 1, wherein when performing the first-stage laser crystallization process and performing the second-stage laser crystallization process, the heat conduction rate of the edge portion of the amorphous silicon island is greater than that of the amorphous silicon The rate of heat conduction in the central part of the island, thereby forming a temperature gradient. 8. The method as claimed in claim 7, wherein the laser crystallization process in the first stage is used to solidify the edge portion of the amorphous silicon island after reaching a completely molten state, and then from the edge portion of the amorphous silicon island At least one seed crystal grows laterally toward the central portion of the amorphous silicon island to form at least one large crystal grain. 9. The method as claimed in claim 7, wherein the laser crystallization process in the first stage is used to make the central part of the amorphous silicon island reach a completely molten state and then nucleate uniformly, so that it cannot be effectively recrystallized and maintained in an amorphous state. crystalline silicon structure. 10. The method as claimed in claim 9, wherein the laser crystallization process in the second stage is used to make the central part of the amorphous silicon island reach a nearly completely molten state, so that the central part of the amorphous silicon island grows into a normal grain. 11. The method of claim 1, wherein the second-stage laser crystallization process is used to increase a process window of the first-stage laser crystallization process. 12. The method according to claim 1, wherein the first energy density is greater than the super lateral growth energy density of the amorphous silicon island, and the second energy density is not greater than the super lateral growth energy density of the amorphous silicon island . 13. The method of claim 12, wherein the first energy density is greater than the amorphization energy density of the amorphous island. 14. The method of claim 12, wherein the second energy density is greater than a near complete melting energy density of the amorphous silicon islands. 15. The laser crystallization method as claimed in claim 1, wherein the method for forming an amorphous silicon island comprises the steps of: forming an amorphous silicon film on the substrate; performing a photoetching process to etch the amorphous silicon film into at least one amorphous Crystal silicon island; Wherein the laser crystallization process in the first stage, the first energy density is greater than the amorphous silicon island energy density; and In the second stage of the laser crystallization process, the second energy density is between the almost complete melting energy density of the amorphous silicon island and the super lateral growth energy density. 16. The method of claim 15, wherein the substrate comprises a glass substrate, a quartz substrate, or a plastic substrate. 17. The method of claim 15, wherein the method of forming the amorphous silicon film comprises a low pressure chemical vapor deposition process, a plasma assisted chemical vapor deposition process, and a sputtering process. 18. The method of claim 15, wherein the laser is an excimer laser. 19. The method of claim 18, wherein the excimer laser comprises XeCl laser, ArF laser, KrF laser or XeF laser. 20. The method of claim 15, wherein when performing the first-stage laser crystallization process and performing the second-stage laser crystallization process, the heat conduction rate of the edge portion of the amorphous silicon island is greater than that of the amorphous silicon island The heat conduction rate in the area of the edge portion of the silicon island, thereby forming a temperature gradient. 21. The method as claimed in claim 20, wherein the first-stage laser crystallization process is used to make the edge portion of the amorphous silicon island solidify first after reaching a completely molten state, and then the edge portion of the amorphous silicon island The at least one crystal seed in the amorphous silicon island grows laterally to a region other than the edge portion of the amorphous silicon island, and recrystallizes into at least one polysilicon crystal grain. 22. The method as claimed in claim 20, wherein the laser crystallization process in the first stage is used to make the region of the edge portion of the amorphous silicon island uniformly nucleate after reaching a completely molten state, so that it cannot be effectively recrystallized and maintained in the amorphous silicon structure. 23. The method as claimed in claim 22, wherein the laser crystallization process of the second stage is used to make the region of the edge portion of the amorphous silicon island reach a nearly completely molten state, thereby making the edge portion of the amorphous silicon island The regions grow into normal grains. 24. The method of claim 15, wherein the second-stage laser crystallization process is used to increase a process window of the first-stage laser crystallization process.

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